The present invention relates to an improved sheet feed apparatus for feeding, for example, an original document to be electrostatically copied onto a transparent platen and subsequently feeding the document into a discharge tray after an imaging exposure is completed.
A basic sheet feed apparatus of this type is disclosed in U.S. Pat. No. 4,023,791 which comprises a plurality of parallel endless belts disposed closely above a glass platen. A document in sheet form is inserted into an inlet and fed by the belts over the platen into engagement with a stopper, at which time the belts are stopped. After an imaging exposure, the stopper is retracted and the belts driven to feed the document off the platen to a discharge tray provided at an outlet. Rollers are provided at various positions to urge the belts into pressing engagement with the document to enhance the transport thereof across the platen.
This apparatus, although generally effective, suffers from a disadvantage where thin documents, such as letters on international airmail stationery, are to be copied. In order to ensure that the document will be carried to the stopper, the belts are allowed to overtravel due to their inertia for a brief distance after the document engages the stopper. For thick documents which are relatively rigid, no problems are encountered since the belts slidably overtravel the document. However, for thin documents, the combination of the frictional force between the belts and document and the low weight of the document cause the document to continue to move after engagement with the stopper. The result is that the leading edge of the document is jammed against the stopper and in some cases torn. At best, the leading edge of the document is merely wrinkled and the resulting copy is out of register with the image area of the document.
In this prior art apparatus the pressure force of the rollers must be great enough to ensure that the belts will feed a thick, heavy document to the stopper. In other words, the belts must not slide over the document until the document engages the stopper. However, such a high pressure force causes thin documents to jam against the stopper in the manner described. Thus, the prior art apparatus cannot accommodate both thick and thin documents without transport failure. Although a switch means may be provided to selectably vary the pressure force of the rollers in accordance with the type of document, such a switch means is a nuisance for the apparatus operator and may frequently be neglected, with the result of a transport failure and possible damage to a valuable document. Although the prior art apparatus greatly increases the copying speed where a large number of sheet documents are to be copied by eliminating the time required to open and close a presser plate for copying each docunent, it has not been heretofore perfected.
Another problem is with static electricity induced on the document by sliding movement of the belts thereover. Such static electricity frequently prevents documents from being neatly stacked in the discharge tray.
Where thick documents are copied, it frequently occurs that the documents rebound upon abutment with the stopper. A lateral movement of the document occurs during rebound due to lateral movement of the belts, and the copy is out of register with the document image area in the lateral direction.
Yet another problem occurs in de-energizing the belt drive means at precisely the right time before the document engages the stopper. If the drive means is de-energized after the document engages the stopper, the document will jam against the stopper. If the drive means is de-energized too soon, the belts will not carry the document completely to the stopper.